
Are there any other niche topics in astronomy that I may find deeply interesting?
For various topics, I would look at:
Introduction to Cosmology by Ryden for cosmology at the undergraduate level.
Cosmology by Weinberg.
The Exoplanet Handbook by Perryman.
An Introduction to Modern Stellar Astrophysics by Carroll & Ostlie.
Particle Astrophysics by Perkins.
Modern Statistical Methods for Astronomy by Feigelson.
Statistics, Data Mining, and Machine Learning in Astronomy by Ivezic.
I can post more in other topics if anyone is interested.
If you want something with more depth, I highly recommend Night Sky from St. Martin's Press. It's a little 300 page field guide that will give you a comprehensive introduction to the heavens and basic astronomy and provides you with enough data to go out observing with. The first quarter of the book is devoted to introducing the fundamentals of astronomy from the celestial sphere, to coordinate systems, to telescope designs, to the magnitude system, to variable stars, and stellar evolution. The rest of the book contains star charts, detailed descriptions of each of the constellations and interesting objects therein, and information on the planets. The only problem is that the book is a bit old, so the tables of data (sunset times, positions of the planets, etc.) are now out of date. Nevertheless, it's well worth buying.
If you would like a more serious introduction to astronomy and know some basic physics and calculus, you can do no better than to read Introduction to Modern Astrophysics by Carroll & Ostlie (affectionately known as BOB -- the Big Orange Book). It's about 1500 pages long and covers absolutely everything in astronomy. It's clear, comprehensive, and will likely tell you everything you need to know.
Further reading: Ch. 13, Carroll & Ostlie, Introduction to Modern Astrophysics